During the last twenty years, the regenerated iron oxides (RIO) produced from hydrochloric acid waste liquid generated from steel pickling have been recognized as an important source of raw material for preparing products containing iron oxides such as iron-oxide based catalysts and high-grade ferrites. The raw material cost savings associated with the substitution of a low cost, regenerated iron oxide for most of the conventional commercial sources of iron oxides are substantial.
One drawback of utilizing regenerated iron oxides from hydrochloric acid waste liquids generated from steel pickling is that the ferric oxides inevitably contain chloride ions, typically in the range of about 0.1 to 0.5% measured as grams of chloride per 100 grams of regenerated iron oxide. In the field of ethylbenzene dehydrogenation catalysts, it is known that the small amounts of residual chloride present in all regenerated iron oxides affect catalyst performance adversely. For the ferrite producers, the chloride ion present in the regenerated iron oxide is not only corrosive to the ferrite manufacturing installations but also injurious to the quality of the ferrite produced.
Reducing the chloride-ion content in regenerated ion oxides by an economical procedure, without significantly changing the ferric oxide quality is believed to be one of the most critical issues for the development of regenerated iron oxides as commercially useful raw materials.
Japanese patent application J59,050,031, published Sep. 13, 1992, discloses a method for purifying iron oxide from waste hydrochloric acid pickling solution containing ferric chloride by treatment with ferrous sulfate, ferric sulfate or ammonium sulfate solution. The iron oxide purified by this prior art method still contains a substantial amount of chloride. Moreover, it is time consuming to dissolve these salts in water. Furthermore, the raw material costs for these sulfate salts are relatively high.
Suganuma, et al. in 4th International Conference on Ferrites, Advances in ceramics, vol. 15, 81-85 (1986), reported a method for removing chloride ions from hematite powders by heating the ferric oxide in air with the addition of sulfur dioxide and/or moisture. However, sulfur oxide is a toxic gas which is unhealthy and environmentally undesirable.
Thus, there remains a need for a commercial process which will substantially reduce the chloride contents in regenerated iron oxides (RIO) at a low cost without significantly changing the quality of the iron oxides.